Relatively simple theoretical procedures enable the pressure loss of the simpler forms of baffle used for flame stabilisation in combustion chambers to be computed, while tests made under non-burning conditions are available for ad hoc testing of the geometrically more complicated forms. Both these methods, with the appropriate corrections for the momentum changes due to burning, have in the past proved suitable for the prediction of the drag of aero-engine combustion chambers under operating conditions. Recently, however, it has been found that under certain conditions, associated especially with very high rates of heat release, the drag of combustion systems can become very much greater than would be predicted either from simple theoretical considerations or from conventional “cold” tests, and vary in an unexpected manner with the normal flow parameters.